fix(#303): Update README with compatibility info

Author: wooldridge

README.md

@@ -15,7 +15,9 @@ from Node.js applications.
 
 ## Status
 
-Release 2.0.1 of the MarkLogic Node.js Client API
+Release 2.0.1 of the MarkLogic Node.js Client API, compatible with MarkLogic
+versions 8 and 9. To use the new features available in 2.0.1, you will need to
+run MarkLogic version 9.
 
 ## New Features in Release 2.0.1